Semiconductor light amplification devices are well known. Such a device is described in an article entitled "Coherent Light Emission from p-n Junctions" published in Solid state Electronics, vol. 6, page 405, 1963. This article describes a laser diode comprising a single semiconductor crystal having a p-doped region and an n-doped region about whose plane boundary, or junction, the population of p and n carriers, i.e., holes and electrons, has been depleted by recombination. That depletion region is also known as the active region. Injection of current into the crystal in a direction transverse to the junction provides additional p and n carriers, which recombine within the active region, emitting one quantum of light energy, or photon, at each such recombination. As a rule, all the emitted photons will have nearly, but not exactly, the same wavelength within a small range characteristic of the semiconductor. By virtue of the crystal's large index of refraction, the cleaved ends, or facets, of the crystal perpendicular to the active region reflect a substantial part of the emitted light, typically thirty percent, back into the active area. As the current injected into the diode is increased, accompanied by a higher rate of photon emission, photons in the active region begin to stimulate carrier recombinations whose consequent emitted photons have the same wavelength and phase as the stimulating photons. The photons fed back to the active region by reflection enhance that process and, at the same time, the resonances of the cavity formed by the opposed reflecting facets promote the selection of discrete wavelengths. Increasing current soon crosses a threshold beyond which coherent emitted light at one or a few such wavelengths increases rapidly. Such coherent light is a laser beam. As is well known, laser beams have found utility in a wide variety of areas, e.g., in the medical and communications fields.
It has been characteristic of laser diodes that the power or intensity they generate is low. To mitigate this limitation, it is well known to optically couple an optical amplifier with the laser diode. Such an optical amplifier is very similar to a laser diode with the exception that the facets of the active region are not reflecting. By means of stimulated photon emission, the optical amplifier raises the power or intensity of the laser light emitted through its output facet without interfering with the coherence properties of the light.
Two serious problems associated with semiconductor diode optical amplifiers severely limit the amplification and the power they can achieve. The first is gain saturation resulting from the interdependence of power density, within the laser beam traversing the optical amplifier, and the electrical power, represented by the injected carriers. At a given electrical pump rate, there is, in the absence of a laser beam, a fixed carrier density in the active region. As the power density of a laser beam traversing the amplifier increases, it reduces the carrier density and, therefore, limits or saturates the gain. To avoid gain saturation, the carrier density must be held constant. The second problem is degradation and catastropic damage of a facet that occurs when the internal power density incident on the facet exceeds certain limits. The present invention overcomes both of these problems.